Devices of this kind are known from EP-A1-0316498 and DE-A1-3038660.
The device according to EP-A1-03136498 comprises a solid cylindrical body which serves as a carrier and at the same time as a mounting base and which has mutually parallel end faces; through which a pin is passed co-axially to the axis of the cylinder and perpendicularly to the end faces so that it projects perpendicularly out of one end face of the carrier. At a distance from the carrier, the pin passes co-axially through a cylindrical seismic mass which similarly comprises end faces perpendicular to the axis of the cylinder. A piezo-electric transducer, which likewise co-axially surrounds the pin, is clamped between the carrier and the seismic mass by means of the pin, which acts as a tightening pin. The pin is slender and resilient and has appreciable play in the transducer. Bending stresses, which in the transducer cause an increase in pressure on one side of the pin and a decrease in pressure on the other, occur under mass inertia forces which load the seismic mass transversely to the axis of the pin. The transducer is provided with divided electrodes which are arranged and connected in correspondence with the pressure stresses arising in push-pull, so that the changes in charge in the electrodes arranged in push-pull are added to provide bending stresses, and the pyro-electric effects caused by heat flow between the carrier and the seismic mass through the transducer compensate for each other.
Devices for the measurement of acceleration should preferably be sensitive to only certain types of acceleration, e.g.: translational or angular, and the axis of acceleration, the acceleration, angular acceleration) and axis of acceleration, the principal axis. However, this is not adequately so in the known device. The known device is a so-called transverse acceleration pick-up, which should respond only to accelerations perpendicular to the axis of the pin. However, because of the need for freedom of tilting of the seismic mass, angular accelerations and linear, accelerations perpendicular to the principal axis have such a strong influence on the measurement that undesired side effects result and in spite of compensation for pyro-electric effects the device is not usable as an accurate accelerometer, but serves virtually only as a sensor.
The known device according to DE-A1 30 38 660 comprises a carrier as well as a seismic mass, which consists of one piece and is so connected with the carrier that it is pivotable relative to the carrier about an elongate bending line. The centre of gravity of the seismic mass and the bending line determine a plane. Arranged at the seismic mass perpendicularly to this plane as well as parallel to the bending line is a piezo-electric platelet which is firmly connected at its ends remote from the plane with the piezo-electric mass and, in the centre therebetween and at the level of the plane, with the carrier. The platelet is polarised in opposite directions parallel to the plane, as well as perpendicularly to its wide sides, which are disposed above and below the central fastening region and parallel to the bending bearing, and is covered by separate electrodes. A third electrode extends on the other side of the platelet and between the fastening regions at the ends. The platelet is compressed on one side of the plane and extended on the other side, thus stressed in shear, by accelerations acting perpendicularly to the plane. The electrodes are so connected together that the piezo-electric effects are added, and these piezoelectric effects are compensated by the pyro-electric effects, which are brought about by heat flow from the fastening point at the carrier through the platelet to the fastening places at the seismic mass.
The transverse compression excitation (k.sub.31), which is present in the afore-mentioned known device, has the disadvantage of a very low mechanical rigidity. The piezo-platelet can break easily and is not clamped over a large area. It is intended that the known device shall measure accelerations perpendicularly to the afore-mentioned plane, but it actually functions as an angle pick-up about the axis of the bending bearing and is thus also sensitive to accelerations transversely to the intended measurement axis to an appreciable degree.
Moreover, a piezo-electric acceleration pick-up with compression excitation and comprising a cylindrical carrier arranged on a base for fastening to the test piece, a seismic mass plugged axially on the carrier and a disc-shaped piezo-electric element disposed under a mechanical bias, is known from DE-OS 30 29 847. The piezo-electric disc excited by compression in direction of the acceleration to be measured is disposed under bias by an arrangement of a rivet, an elastic element and a seismic mass arranged at the axial end of the carrier. This known acceleration pick-up is sensitive to the influence of external temperature. The piezo-electrical element is disposed--apart from electrodes and insulating elements therebetween--directly on the base, so that a heat flow through the piezo-electric element is present in the case of a temperature difference between the base and the seismic mass. The heat flow causes a charge separation, thus a pyro-electric effect at the piezo-electric element similarly to the case of compression or shear. The sign and extent of the pyro-electric effect depend on the polarisation of the piezo-electric element concerned, thus on whether the heat flow takes place in or against the direction of the potarisation. Temperature gradients and heat flow between the base and the seismic mass are virtually unavoidable in many measurements and falsify the measurement results in the known device due to the appreciable pyro-electrically caused charge displacements at the piezo-electric element. The connection of the piezo-electric element directly with the base is also disadvantageous to the extent that a deformation of the base, as for example can arise due to mechanical stressing or due to temperature deformation, acts directly on the piezo-electric element itself influencing its piezo-electric effect, so that the known acceleration pick-up also has a low insensitivity to expansion of the base. This applies also to the known device according to EP-A1-03 16 498 discussed above. However, acceleration pick-ups should have a high base expansion insensitivity.
A further known piezo-electric acceleration sensor explained in DE-0S 30 19 551 comprises a carrier which stands on a base and which, for improvement of temperature constancy, has substantially the same thermal coefficient of expansion as the piezo-electric elements subjected to shear excitation thereat. Although acceleration sensors of that kind have little sensitivity to temperature differences, they have, however, only a low mechanical rigidity, just as the above-described known acceleration pick-up according to DE-A1 30 38 660. Seismic masses by their nature develop large inertia forces and shear forces in acceleration pick-ups with shear excitation and therefore damage their fastening to the carrier or damage the carrier itself. Acceleration pick-ups with shear excitation are also unsuitable for measurement of high-frequency oscillations, since the carrier arrangement and seismic masses coupled thereto easily get into resonance or develop natural oscillations.